This invention relates to a location and ranging system useful as a surveillance system for enclosed areas and, in particular, to such a system using a pair of leaky coaxial cables, one connected to a transmitter and the other to a receiver.
It is well known that there is a requirement for detecting unauthorized movement either entering into, or departing from, large enclosed areas such as prisons, airports, warehouses, freight yards and defence installations. The numerous types of apparatus presently employed in these applications all have serious disadvantages, especially for long perimeters (in excess of 1 mile) during adverse environmental conditions. Heavy rain or snow conditions can disable most optical systems based upon photoelectric sensors. Pressure sensitive devices can be ineffective in cold climates due to the penetration of frost. Both acoustic and seismic sensors are prone to false alarms due to gusts of wind or the proximity of vehicular traffic.
A number of perimeter protection systems are based upon the disturbance of electro-magnetic fields. Some systems rely upon the change of capacitance between two sensing wires. Others rely upon the change of impedance of a two wire transmission line due to the presence of an intruder. Most of these systems have relatively poor sensitivity because they attempt to detect very small changes in a large quantity which usually is a function of the physical deployment of the sensor. This can result in false alarms due to vibration, rain, snow or variations in temperature and humidity.
The use of coupled transmission lines together with digital processing can overcome most of these shortcomings. U.S. Pat. Nos. 3,750,125 and 3,801,976 disclose a system based upon coupled strip lines. In that system, the transmission lines are separated by a fraction of an inch and detection of an object (a wheel of a vehicle or a shoe on a human foot, for example) occurs when the presence of the object caused a change or distortion in the electro-magnetic field coupling pattern. Due to the close proximity of these transmission lines, the zone of protection in such systems is very limited. In addition the construction and installation of these strip lines can be very cumbersome.
Coupling may be defined as "close" or "loose." The customary definition of these two types of coupling may be found in the "IEEE Standard Dictionary of Electrical and Electronics Terms" at page 123. A further definition of "loose coupling" may be noted in the text book "Electrical Engineering Circuits" by Skilling (Wiley, 1957) at page 369. The statement is made that "loose-coupled" circuits have a coupling coefficient in the range between 0.01 and 0.10.
Thus, in the known system the coupling between the transmission lines is close. A study of typical microstrip transmission lines (IEEE Trans MTT-16 No. 12, December 1968 pp. 1021-1027) deals with the question of coupling between closely positioned microstrip transmission lines almost identical to those described in the U.S. Pat. Nos. 3,750,125 and 3,801,976. FIGS. 6-8 (p. 1025) show that the characteristic impedance of the coupled pair is dependent on the strip line spacing. This establishes that lines such as those disclosed in the patents operate under conditions of close coupling. It may further be noted that in U.S. Pat. No. 3,801,976 the patentee states "For use in the present invention, a maximum coupling is again usually selected,".